An accelerometer is an important test component in inertial navigation, inertial guidance, and control testing devices. Both inertial navigation and inertial guidance utilizes the accelerometer's sensitivity to test the movement acceleration of the carrier. Nowadays, accelerometers are widely applied to aviation, navigation, astronavigation, earthquake detection, and precise guidance and control. There are various accelerometers, including pendulum accelerometers, flexure accelerometers, electromagnetic accelerometers, MEMS accelerometers, and optical accelerometers, etc.
Optical accelerometers have many advantages such as resistance to electromagnetic interference, high sensitivity, high SNR (signal to noise ratio), and high stability, etc., and as such, have been a focus of major research directions in the field of accelerometers in recent years. The detection principle of an optical accelerometer is as follows: since the optical signals in the sensitive element (mass block) are modulated by the acceleration to be measured, the optical properties (e.g., optical intensity, phase, or resonant frequency) of the optical signals which are received by an optical detector after being transmitted, reflected, or interfered in the optical circuit are changed; and then the optical signals are fed into a photoelectric detector to obtain the measured physical quantities by means of appropriate demodulation technologies. At present, optical accelerometers that are mainly studied include phase modulation accelerometers and frequency (wavelength) modulation accelerometers. Phase modulation optical accelerometers detect the value of acceleration by detecting the phase change of transmitted light which is caused due to the action of inertial force on the optical sensing element (e.g., optical fiber). Such accelerometers usually have an optical structure such as a Michelson, Maeh-Zehnder, or Fabry-Perot chamber, and detect acceleration by detecting the change of optical intensity after the signal light interferes with the reference light. A main drawback of such accelerometers is if the phase difference between the two optical signals is small, then the change of optical intensity is not obvious, and therefore the detection sensitivity is not high. Frequency modulation optical accelerometers are developed on the basis of phase modulation optical accelerometers. They employ special device structures with periodic frequency selection function, such as optical grating, fiber grating, and resonant ring, etc., and utilize the relation between resonant frequency and inertial force to detect acceleration. When the sensitive element to be measured produces inertial force or displacement during accelerated movement, the displacement of the optical path system at the resonant frequency will change. The value of acceleration can be obtained by measuring the horizontal displacement at the resonant frequency. Since the subtle change in phase difference of signal light is further amplified by enhancement of multiple-beam interference, the detection sensitivity is higher. However, due to the effect of environmental temperature fluctuations and double refraction of waveguide, the resonant spectral lines have horizontal displacement and asymmetrical distribution, which result in severe degradation of detection sensitivity of the device.
According to the system constitution of elements and devices, the optical sensing elements and optical transmission path of existing optical accelerometers are mainly composed of discrete devices such as optical fiber, optical grating, fiber grating, reflector, etc., which have large size, high production cost, and poor system stability, etc. Moreover, optical fiber devices are sensitive to temperature fluctuations, and the splicing loss and polarization effect of optical fiber devices will influence the stability and detection sensitivity of the accelerometer.
The development of technology of integrated optical devices brings a new developing trend to optical sensors. With micro-nano precision machining technology, various optical elements and devices can be integrated on the same substrate, and the discrete functional elements and devices can be connected through optical waveguides, so as to further reduce the size of the optical sensor system. In addition, integrated optical devices have advantages such as high stability, high reliability, simple production process, and more available materials, etc., and can meet the technical demand for development of high-precision optical acceleration sensors. In recent years, full-polymer optical waveguide devices, comprising substrate, cladding and core layer which are made of organic polymer materials, has been a popular focus for research. Optical waveguides having such a structure are insensitive to temperature fluctuations; in addition, since the substrate is made of organic polymer material, it has advantages such as lower elastic modulus, higher sensitivity to stress and strain, higher toughness, and high resistance to fracture, etc., over conventional silicon wafer or quartz substrate, and can be applied to develop high-precision mechanical sensors.